Rowling's final Harry Potter book was published in 2007
JK Rowling has written a short prequel to her popular Harry Potter books.
But the 800-word tale will not be published - instead it will go under the hammer at a charity auction in London next month.
She is one of 13 authors - including Nick Hornby and Doris Lessing - asked to write storycards by Waterstone's for their What's Your Story? auction.
Rowling's story finishes with the handwritten words: "From the prequel I am not working on - but that was fun!"
All 13 cards will be sold without a reserve price. The other authors include Margaret Atwood, Lauren Child, Sebastian Faulks, Tom Stoppard and Irvine Welsh.
Gerry Johnson, managing director of Waterstone's said: "What's Your Story just gets more and more exciting - all these brilliant works by amazing writers and illustrators, every one a small masterpiece, and now this - the prequel to Harry Potter.
"We never dreamed that JK Rowling would donate something so precious, and we're incredibly grateful."
Money raised from the sale will go to English PEN, which promotes understanding through literature, and Dyslexia Action.
In December last year, a new JK Rowling work, The Tales of Beedle the Bard, fetched £1.95m at auction.
Philip Errington, a specialist at Sotheby's, which will run the auction at Waterstone's Piccadilly store on 10 June, said: "Given the enormous interest we have seen in recent times for autograph work by JK Rowling, the prospects for her storycard are good to say the least.
"There is just no telling how high the bidding might go."